1. Field of the Invention
This invention relates generally to log-domain filters and in particular to a biasing system for a log-domain filter having a variable dynamic range window.
2. Background
In wireless communications systems, such as those employing the CDMA IS-95A, -95B, -95C standards, there is typically a wide variation in the amplitude of signals transmitted between the mobile units and the handsets thereof caused by various factors such as slow and fast fading. In many cases, the range in amplitude in such systems can exceed 60 dB. Consequently, the filters used in such systems must have sufficient dynamic range to handle the wide variability of signals which are present. Even in systems employing closed loop power control, such as CDMA wireless communications systems, there are circumstances in which a wide dynamic range filter is required.
The problem is that there are limits in the amount of current or voltage swing current active filters can handle and still remain linear. In traditional filters, such as gmC filters, the dynamic range can only be increased through increased power consumption and die area. For example, traditional methods for increasing the dynamic range involve either the introduction of larger degeneration resisters on the emitters of differential pairs, which is wasteful of die area, or higher bias currents, which is wasteful of power. Furthermore, the range is fixed by the design, and cannot be varied to accommodate signals of widely varying amplitude. Consequently, the power consumption in such filters is always set to accommodate worst-case scenarios, making the consumption wasteful at all other times.
One class of filters that have been proposed in response to the demands presented by modem communication systems is log-domain filters. See, e.g., M. Punzenberger & C. Enz, "A Compact Low-Power BiCMOS Log-Domain Filter", IEEE J Solid-State Circuits, vol. 33, no. 7, pp. 1123-1129, July, 1998; M. Punzenberger & C. Enz, "A 1.2-V Low-Power BiCMOS Class AB Log-Domain Filter," IEEE J Sold-State Circuits, vol. 32, no. 12, December, 1997, both of which are hereby fully incorporated by reference herein as though set forth in full.
Log-domain filters typically comprise a logarithmic integrator sandwiched between a logarithmic compressor at the input, and an exponential expander at the output. An incoming signal is compressed into the log domain, filtered while in the log domain, and then exponentially expanded. Log-domain filters, because they operate on signals in the log domain, can handle signals having large swings in amplitude. Moreover, despite their internal non-linearity, log-domain filters achieve linearity from an overall standpoint.
The problem is that the dynamic range of log-domain filters cannot achieve a dynamic range of greater than about 60-70 dB. The reason is that the noise floor of a log-domain filter, which defines the lower bound of the dynamic range, rises and falls as the compression point, which defines the upper bound of the dynamic range, rises and falls. Consequently, the dynamic range, which is defined as the range between the compression point and the noise floor, is substantially invariant to changes in the compression point. The dynamic range thus remains at its nominal level of about 60-70 dB. Thus, these filters are not suitable for general use in wireless communications systems, particularly wireless communications systems where very wide dynamic range is required.
Switchable RC filters offer a wide dynamic range, but the transfer function thereof is subject to numerous, discontinuous jumps as components switch in and out of the filter during tuning. Such discontinuities render the switchable RC filter unsuitable for general use in wireless communications systems. In addition, such filters consume a large die area.
Other tunable filters comprise gmC filters. However, like log domain filters, gmC filters are limited to a 60-70 dB dynamic range.
Thus, there is a need for a tunable filter which overcomes the disadvantages of the prior art.